Engine having rotary pistons

ABSTRACT

The present invention relates to a mechanism ( 1 ) having rotary pistons ( 21 ), said mechanism comprising an outer enclosure ( 2 ) forming a stator in which a rotary assembly ( 20 ) forming a rotor moves, the rotary assembly ( 20 ) including: a plurality of pistons ( 21 ) forming a hinged polygon, each piston ( 21 ) defining a positive-displacement chamber ( 35 ) with the enclosure ( 2 ), and first guide ( 27, 27   a,    27   b ) that are arranged so as to engage with second guide means ( 7   a,    7   b ) comprising rolling surfaces ( 10   a,    10   b ) so as to drive the polygon to carry out a predetermined movement, the mechanism ( 1 ) being characterized in that it comprises a drawback device ( 50 ) that maintains contact between the guide means ( 27, 27   a,    27   b ) and the rolling surfaces ( 10   a,    10   b ). The invention also relates to an engine and a pump including such a mechanism ( 1 ).

TECHNICAL FIELD

The present invention relates to a rotary piston mechanism as well as anengine and pump using such a mechanism.

BRIEF SUMMARY OF RELATED ART

Rotary piston mechanisms, and in particular in their motor application,have been described in many documents, which define the geometricprinciples of the movement of a polygon articulated at the apices of thesides making it up; each of the sides all having the same length andeach of the apices being in geometric contact with a stator shape.

In particular, documents WO 01/88341 A1, EP 1 295 012 B1 and US2004/0089251 A1 propose different solutions for resolving the problemsof sealing at the apices of the polygon, and transmission of thenon-constant continuous movement of the pistons formed by the straightsegments defined by the adjacent apices of the polygon.

All of these proposals more or less appropriately resolved all or someof the theoretical operating principles of this type of mechanism.

However, the feedback from tests done has shown difficulties for whichno technical solution has yet been proposed.

The difficulties encountered during the exploitation of this type ofmechanism first relate to mechanical tightening problems when it is hotas well as during assembly of the mechanism. The symptoms are blockagesof the hinge assembly, which, despite a careful production, cannot pivotfreely around the various axes making up its mechanism.

Possible compensations by excess play create knocking at the connectingrod assemblies, and pronounced wear of certain points of the mechanism,thereby limiting the lifetime of the pieces thus embrittled.

During analyses, it was also observed that the geometric and algebraicsolutions were describing the ideal geometric locations, but with verysmall dimensions, i.e. solutions that are not very compatible withmechanical play tolerance accumulation phenomena.

Among the cited patents, some propose solutions that separate thesealing functions at the apices from the movement functions specific tothe polygonal structure.

Thus, they propose a device having a movement specific to the polygon,independent of the profile of the stator enclosure inside which theapices of the polygon are in constant sliding contact.

This mechanical approach appears correct and therefore consists ofdetermining the movement of the articulations of the polygons withoutaction by the stator on the apex of the polygon.

In fact, one of the origins of tightening problems comes from imprecisemachining of the stator profile. The decision to make the articulationsof the rotary assembly independent of the stator shape helps resolvesome of the defect problems.

Patent WO 01/88341 A1 proposes a solution describing the chaining offour pistons to one another using connecting rods also performing theappropriate transfer function between the pistons and the transmissionshaft, as well as several mechanical solutions resolving the transferfunction between the tangential speed variations of the pistons towardthe central transmission shaft rotating at a constant speed, inparticular using rollers traveling over a rolling surface installed onthe lateral flanges.

However, after analysis, it was identified that the project as describedwas proposing a hyperstatic configuration that did not allow any playtolerance. In fact, the trapezoid described by the two connecting rodsof the two adjacent pistons, the connecting point of the pistons and theconnecting point on the driveshaft builds a hyperstatic connection. Thistrapezoid is undeformable and cannot allow any variation of the lengthof any one of its sides, as will be the case during the use of themechanism, which will display expansion phenomena. The bearings of thearticulations will be the embrittled members. These parts will wearprematurely while having created internal friction making the mechanismunsuitable for its purpose.

Furthermore, the shape of the connection between pistons does not appearcapable of sustainably bearing the intense pulling, thrust, andcentrifugal forces to which it will be subjected. These bowedcollaborating movable interlocking forms are complex in terms ofexpansions, buttress formation, friction distribution, and their use isas difficult as it is unreliable. The sealing of such pieces is alsomore complex to achieve than the sealing of traditional axles and notvery effective inasmuch as the stressed orientation of the sealingsegment does not correspond to the forces undergone by that piece.

In case of expansion, in cases of temperature variation, this geometricsolution is no longer respected, which results in a mechanical stress onthe connection of the apex of the diamond without the possibility ofabsorbing that stress, which will result in a rupture of the connectionbetween pistons, as well as stresses on the connecting rod assemblies,the latter not being able to transmit the expansions. This willirreversibly cause problems such as, for example, a rupture of aconnecting rod bearing, or the destruction of the segment support deviceor the segment itself, which is already working in a cantilever, whichmay also result in deterioration of the stator profile by burring of itsinner surface.

A machining anomaly, on the rolling surfaces, will result in apositioning error of the apex of the diamond, the sealing piece of whichwill be the only one to react this additional force. This piece will bestressed in its degree of freedom and may no longer pivot and willbreak; this stress may potentially be passed on to the connectionbetween the pistons.

Patent EP 1 295 012 B1 and part of patent US 2004/0089251 A1 alsopropose determining the deformation of the polygon without bearing onthe stator enclosure. This proposed solution uses a set of gears.

However, the sealing problem at the apices of the polygon does notappear to have been resolved.

Lastly, patent US 2004/0089251 A1 proposes a geometric solutionaccumulating the sealing function and a geometric function.

However, this solution does not resolve the problems of machiningimprecisions and wear of certain parts of the mechanism as it is used.

BRIEF SUMMARY

The invention therefore aims to propose a technical solution making itpossible to do away with these operating difficulties due to machiningimprecisions, expansion phenomena due to the temperature or otherfactors, and wear phenomena of the pieces.

To that end, the present invention relates to a rotary piston mechanismcomprising an outer enclosure forming a stator inside which a rotaryassembly forming a rotor moves, the rotary assembly comprising aplurality of pistons forming an articulated polygon, each pistondelimiting a volume chamber with the enclosure, and first guide meansarranged to cooperate with second guide means comprising rollingsurfaces, so as to stress the polygon to perform a predeterminedmovement, wherein the mechanism comprises a return device maintainingcontact between the guide means and the rolling surfaces.

The first and second guide means allow the pistons to follow apredetermined direction so as to limit the action of the enclosure onthe apices of the polygon while the return device makes it possible tooffset the machining defects of the guide means as well as the expansionand wear of those elements occurring during operation of the mechanism.

According to one embodiment, the return device is supported by therotary assembly.

According to one embodiment, the return device comprises a plurality ofdampers positioned between a stationary point of each piston and amovable fastening point of each of the first guide means, such as theguide roller axis of rotation.

This arrangement makes it possible to offset the wear of each of theguide rollers individually.

Using several rollers makes it possible to distribute the forces appliedon the guide surfaces. Furthermore, these rollers have a small bulk andlimit the friction on the guide surfaces due to their degree ofrotational freedom.

According to one embodiment, the guide rollers are conical.

It is easier to design guide rollers whereof the surfaces are parallel;however, a conical configuration of the guide rollers makes it possibleto increase the bearing surface of the rollers relative to a flatconfiguration and prevents the guide rollers from deviating from theirpath.

According to this same embodiment, the second guide means are rollingsurfaces with an equal slope.

According to another embodiment, the return device is supported by theouter enclosure.

This arrangement makes it possible to place the return device atstationary points and limit the number thereof.

According to one embodiment, the second guide means are supported bylateral flanges positioned coaxially to the outer enclosure and servingto close the latter part.

This arrangement makes it possible to take advantage of the ease ofassembling and disassembling the flanges used to close the outerenclosure so as to position the second guide means therein. It is alsopossible to consider manually pre-adjusting the orientation of saidsecond adjustment means using a suitable device before closing of theenclosure by the flanges.

According to one embodiment, the second guide means comprise twosectors, the first of which is secured to the stator enclosure and thesecond of which is translationally movable relative to the first, areturn device being positioned between a stationary point of the firstsector and a movable point of the second sector of each of the secondguide means.

This arrangement makes it possible to be able to modify the eccentricityof the guide surfaces, which assume a substantially ellipsoidal form,and thus makes it possible to offset the general wear of the guidesurfaces as well as their machining defects.

According to one embodiment, a segment support device is pivotablymounted along a single axis on each of the apices of the polygon.

This arrangement allows the segment support device to adapt to the shapeof the enclosure and to present the segments at all times in a directiontransverse to the wall of the stator in which they evolve, which reducesthe wear of the segments and imparts better sealing to the volumechambers.

Advantageously, a segment support device comprises a device for pressingsegments on the stator enclosure.

The purpose of this arrangement is to produce a continuous slidingcontact of the segments on the inner surface of the single-piece body soas to offset the machining defects thereof and therefore further improvethe sealing between the volume chambers.

According to one embodiment, at least one piston is connected by asingle connecting rod (24) or by several connecting rods (241, 242)having the same hinge pins as the transmission shaft (40).

The use of connecting rods is geometrically easier to design than a setof gears, for example, and will be a source of less play due to thereduced number of elements and connections.

In this embodiment with connecting rods, the invention consists ofkeeping only one connecting rod per piston, for example that defined asrear in patent WO 01/88341 A1 in the case of rotation in the indirecttrigonometric direction or in the clockwise direction.

The transfer function between the pistons and the engine shaft is kept;the expansions are no longer problematic, as an expansion will onlycreate an imperceptible rotation of the rotary assembly relative to areference position.

According to one embodiment, two adjacent pistons are connected to oneanother by a pivot link with an axis parallel to the axis of rotation ofthe rotary assembly.

This configuration makes it possible to obtain a connection of twoadjacent pistons that is both very solid and flexible, and is also easyto produce, as it only comprises a single pin fitted into both ends ofthe two pistons.

According to one embodiment, the rolling surfaces are turned toward theaxis of rotation of the rotary body.

According to one embodiment, the rotary assembly comprises a lubricationcircuit mounted in a closed circuit.

This arrangement makes it possible to use a so-called “dry sump”lubrication system with all of the advantages thereof, in particularmore significant and targeted lubrication at the critical locations, alowered center of gravity of the mechanism due to the lack of an oil panunder the mechanism, a reservoir outside the mechanism that is not verybulky, and the possibility of incorporating a heat exchanger for coolingoil in the lubrication circuit.

According to one embodiment, the distance between the connecting pinbetween two adjacent pistons and the hinge pin of the segment supportdevice is constant.

This arrangement makes it possible to define a particular stator profilecorresponding to this geometric constraint that will only give thesegment holders and segments their sealing function between volumechambers.

According to one embodiment, the distance between the connecting pinbetween pistons and the hinge pin of the segment support device isvariable.

This arrangement makes it possible to obtain maximal volume ratiosbetween chambers to the extent that the segment support devices andsegments still perform their sealing function.

According to this embodiment, the segment support device comprises acomplementary device for pressing segments on the stator, such as ahydraulic piston.

This arrangement offers the possibility of being able to still furtherincrease the volume ratios between chambers by offering the segmentsupports and segments the possibility of ensuring sealing of the volumechambers despite substantial spacing of the wall of the stator forcertain positions of the pistons.

The present invention also relates to a pump comprising a mechanism asdescribed above as well as an engine using the same mechanism in its usefor producing mechanical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

In any event, the invention will be well understood using the followingdescription, done in reference to the appended diagrammatic drawings,showing, as non-limiting examples, several embodiments of a mechanismaccording to the invention.

FIG. 1 shows a summary projection diagram of the profile of themechanism according to the invention.

FIG. 2 shows a summary projection diagram of the profile of themechanism with a first embodiment of the arrangement of the playcompensating device.

FIG. 3 shows a summary projection diagram of the profile of themechanism with a second embodiment of the arrangement of the playcompensating device.

FIG. 4 shows the arrangement of three pistons in perspective viewpositioned outside the stator enclosure.

FIG. 5 is a perspective view of a second guide means with its playcompensating device according to the embodiment illustrated in FIG. 3.

FIG. 6 is a perspective view of the inner portion of a flangeincorporating the second guide means with its play compensating deviceillustrated in FIG. 5.

FIG. 7 is a perspective view of the outer portion of the flangeillustrated in FIG. 6.

FIG. 8 shows a cross-sectional and profile view of the mechanismaccording to the invention.

FIG. 9 shows a cross-sectional view of a hydraulic damper of themechanism according to the invention.

DETAILED DESCRIPTION

As illustrated in FIGS. 1, 2, 3 and 8, a rotary piston 21 mechanism 1includes an outer enclosure 2 forming a stator inside which a rotaryassembly 20 forming a rotor moves.

All of the reasoning described hereafter is valid for a polygon with napices. The following reasoning is based on an embodiment with a polygonhaving four apices, or with four pistons forming an articulateddeformable diamond, this example not being limiting.

The outer enclosure 2 in the embodiment illustrated in FIGS. 1, 2, 3 and8 includes a single-piece body 3 generally made from steel, forming avolume with two identical ellipsoidal bases, in which anellipsoid-shaped through cavity 5 is bored. The inner surface 4 of saidcavity 5 is advantageously glazed so as to give it a surface statesatisfactory for the applications of the mechanism 1.

The outer enclosure 2 also includes two lateral ellipsoid-shaped flanges6 a, 6 b, shown in FIGS. 6 and 7, closing the through cavity 5 at eachof the two ellipsoidal bases of the single-piece body 3. Each of saidflanges 6 a, 6 b is screwed to the single-piece body 3 of the enclosure2 and incorporates sealing means such as an O-ring positioned in agroove situated on the joint of the flanges 6 a, 6 b with thesingle-piece body 3.

Each of the flanges 6 a, 6 b comprises a bearing 8 a, 8 b centered toallow the rotation of the transmission shaft 40 along the axis 30 of thebearings 8 a, 8 b.

Each of these bearings 8 a, 8 b is respectively secured by screws 18 ato each of the flanges 6 a, 6 b and comprises two cushions on the innercontour 18 b thereof as well as an oil seal flush on the outer surface18 c of each of the bearings 8 a, 8 b so as to perform oil sealingbetween the barriers 8 a, 8 b in the transmission shaft 40.

As illustrated in FIG. 6, guide means 7 a, 7 b having a volume shapewith substantially ellipsoidal bases 19 a generating a first rollingsurface 10 a and a second rolling surface 10 b of revolution on theouter contour 19 b of these same substantially ellipsoidal bases 19 a,are positioned coaxially to the alignment axis 30 of the two flanges 6a, 6 b and oriented towards the inside of the enclosure 2. They arefastened using screws 9 a passing through each of the bearings 8 a, 8 bfrom the outside towards the inside of the flanges 6 a, 6 b and beingscrewed on threaded bores 9 b positioned opposite the screws 9 a on eachof the guide means 7 a, 7 b.

The substantially ellipsoidal guide means 7 a, 7 b are coaxial and havea slight rotational offset along their axis 30. Each of said guide means7 a, 7 b has an ellipsoidal central opening 11 whereof the small axishas a size larger than the diameter of the bearings 8 a, 8 b so as notto bother the rotation of the transmission shaft 40.

According to one embodiment, these guide means 7 a, 7 b are made in asingle piece having a predefined shape not evolving during use of themechanism 1.

According to another embodiment illustrated in FIG. 5, these guide means7 a, 7 b comprise two sectors 71 and 72 separating the substantiallyellipsoidal volume into two volumes at the small axis thereof, therebycreating four coplanar and distinct cutting surfaces 12 a, 12 b, 12 cand 12 d on each of the sectors 71 and 72, each of the surfaces of asame sector 71 being opposite a surface of the other sectors 72 whilebeing parallel thereto.

The sector 71 is fastened to the bearings 8 a, 8 b using screws 9 atightening in the threaded bores 9 b, the second sector 72 is slidinglymounted in translation relative to the sector 71. The two sectors 71 and72 have a first mechanical link 13 formed by a pin positioned betweentwo first surfaces 12 a, 12 b opposite the sectors 71 and 72. This pin13 is secured on the surface 12 a and slides translationally in a borepositioned on the surface 12 b.

A second mechanical link 14 is formed in the same way between the othertwo opposite surfaces 12 c, 12 d. However, the axis 14 of this link issurrounded by conical washers 15 acting in compression, such as aspring, and acting as a play compensating device 50.

In order to ensure the continuity of the deformable rolling surfaces 10a and 10 b to adapt to the relative translational movements of the twoadjacent edges of the two sectors 71 and 72, positioned on each of theedges of the two sectors 71 and 72 of each of the guide means 7 a and 7b is a joint 16 of the cantilever comb expansion type. These joints 16are made up of a set of identical parallelepiped teeth 17 a, spacedapart by a same distance and coming transversely from the cuttingsurfaces 12 a, 12 c of the sector 71, and by another set of teeth 17 acoming transversely from the cutting surfaces 12 b, 12 d of the sector72 respectively located opposite the cutting surfaces 12 a, 12 c of thesector 71, each set of teeth 17 a fitting into the spaces 17 b betweenteeth 17 a created by the other set of teeth 17 a.

The rotary assembly 20 forming a rotor is made up of four pistons 21forming a deformable diamond. The pistons 21 have a convex shape on theouter surface 21 a thereof and a planar shape on the inner surface 21 bthereof.

Each of the adjacent pistons 21 is solidly connected using a pivot linkmade up of a pin 22 passing transversely through the ends of twoadjacent pistons 21.

Each piston 21 has a parallelepiped recess 23 whereof two walls 23 a, 23b are positioned transversely to the pins 22 ensuring the connection ofthe adjacent pistons 21.

In this recess 23 and between these two walls 23 a, 23 b, a connectingrod 24 is positioned here made up of two connecting rods 241, 242positioned side-by-side and separated by a space 24 c. These twoconnecting rods 241, 242 are maintained by a pivot link formed by a pin25 passing transversely through the two walls 23 a, 23 b as well as thehead 24 a of each of the two cylindrical connecting rods 241, 242. Thispin 25 is thus positioned parallel to the pins 22, linking the adjacentpistons 21 to one another.

Each of the pistons 21 also comprises two through openings 26 a, 26 b,which are also parallel to the axes 22, and made on the lower portion ofthe piston 21 on either side of the recess 23. These two openings 26 a,26 b are intended each to receive a first guide means 27, illustrated inFIGS. 1, 2, 3 and 8 by rollers 27.

Each of the pistons 21 only comprises two rollers 27 each positioned onthe lateral surfaces 21 c, 21 d of the piston 21. In the direction ofrotation of the rotary assembly 20, a front guide roller 27 a and a rearguide roller 27 b are distinguished whereof the axes of rotation are oneither side of the recess 23 coaxial to each the openings 26 a, 26 b.The piston 21 has a suitable profile on either side allowing therotation of each of said two rollers 27 a and 27 b.

On the same end of their outer surface 21 a, each piston 21 has asemicircular transverse notch 28 on the width of the piston 21. Insidethat notch 28, there is a segment support device 29 having, like thenotch 28, a semicircular shape allowing it pivot in the notch 28 alongthe hinge pin 32 of the segment support device 29.

On said segment support device 29, two radial segments 31 are positionedprotruding from the convex surface 21 a of the piston 21. Each of thesesegments 31 is pushed toward the outside of the device 29 by the actionof a spring 33 positioned in the device 29.

In the embodiment illustrated in FIGS. 1, 2, 3 and 8, the distancebetween the linking pin 22 of two adjacent pistons 21 and the hinge pin32 of the segment support device 29 is constant.

As illustrated more precisely in FIG. 8, each of the connecting rods 24is, as already mentioned above, made up of two connecting rods 241, 242positioned side-by-side and separated by a space 24 c. Each of these twoconnecting rods 241, 242 is connected to a transmission shaft 40 bymeans of the pivot link made by a pin 34 passing transversely throughthe foot 24 b of each of the cylindrical connecting rods 241, 242 on theone hand, and on the other hand, a hub 41 of the transmission shaft 40passing in the space 24 c positioned between the feet 24 b of the twoconnecting rods 241, 242.

It is understood that these two connecting rods 241, 242 could beconnected to one another to form a single mechanical part.

According to the embodiment illustrated in FIG. 2, the two guide rollers27 a and 27 b present on the lateral surfaces 21 c, 21 d of each piston21 are connected thereto by means of a return device 50 or playcompensating device such as hydraulic dampers 50, illustrated in FIG. 9.

A hydraulic damper 50 is made up of a piston-cylinder assembly whereofthe internal leaks are calibrated. The piston 51 is pierced at thecenter thereof with a calibrated diameter and the ball 53, provided witha spring 54 acting as a check valve, imposes the passage direction ofthe oil through the calibrated space 55 between the piston 51 and thecylinder 52.

In this embodiment, a stationary connection by means of a screw (notshown) is made between the piston 21 and the first fastener 56 of thehydraulic damper 50 and a pivot link is made by means of a pin (notshown) between the second link 57 of the hydraulic damper 50 and theaxis of rotation 27 c of a roller 27.

Once the rotary assembly 20 is assembled, it is inserted inside thesingle-piece body 3 and enclosed inside the stator enclosure 2 duringmounting of the lateral flanges 6 a, 6 b.

Once mounted, the rotary assembly 20 delimits, with the stator enclosure2, volume chambers 35.

A closed oil circuit (not shown) is arranged in the various parts of therotary assembly 20 so as to lubricate the contact areas between thedifferent moving pieces relative to one another, as is the case for thepins 25 and 34 respectively maintaining the head 24 a and the foot 24 bof the connecting rods 24. This so-called dry sump circuit includes anoil pump as well as an oil reservoir, which are both positioned outsidethe stator enclosure 2. A heat exchanger is also arranged in the oilcircuit so as to cool it.

It should be noted that the embodiment comprising arranging a hydraulicdamper 50 between each piston 21 and roller 27 of the rotary assembly 20and positioning guide means 7 a, 7 b in two sectors 71 and 72 capable ofmoving in translation relative to one another does not prohibitembodiments comprising using only the hydraulic dampers 50 between eachpiston 51 and roller 27 of the rotary assembly 20 or the guide means 7a, 7 b.

Once the mechanism 1 is mounted, it can be used as an engine so as toproduce torque on its transmission shaft 40, or as a pump by drivingthat same transmission shaft 40 using a coupling connecting it to anexternal engine.

Of course, both of these applications require developments of thesingle-piece body 3 so as to create intake and exhaust pathways thereinfor pump, external combustion engine, or steam expansion engineapplications, as well as lighting for internal combustion engineapplications.

During the rotation of the rotary assembly 20 inside the statorenclosure 2, the front rollers 27 a of each of the pistons 21 follow thetrajectory of the rolling surface 10 a of the guide means 7 a, while therear rollers 27 b of each of the pistons 21 follow the trajectory of therolling surface 10 b of the guide means 7 b.

The apices of the diamond formed by the four pistons 21 thus follow thetrajectory of the inner surface 4 of the single-piece body 3 of thestator enclosure 2 without bearing on that same surface 4.

The segments 31 by means of the spring 33 present in the segment supportdevice 29 ensure continuous sliding contact on the inner surface 4 ofthe single-piece body 3 of the stator enclosure 2.

The connection of the segment support device 29 in its notch 28guarantees the segments 31 radial contact with the inner surface 4 ofthe single-piece body 3 of the stator enclosure 2, which makes itpossible to reduce the wear thereof and preserve good sealing of thevolume chambers 35.

During the operation of the mechanism 1, expansion phenomena may appear,the transfer function between the pistons 21 and the transmission shaft40 is preserved and the expansions will only create an imperceptiblerotation of the transmission shaft 40 relative to a reference position.

During extended operation with poor lubrication capable of causing wearof one of the rolling surfaces 10 a, 10 b or several rollers 27, thehydraulic dampers 50 arranged on each of the rollers 27 will offset thatwear at all times by providing continuous contacts between the rollers27 and the two rolling surfaces 10 a and 10 b.

In the case of overall wear of the rollers 27, the rolling surfaces 10 aand 10 b in the embodiment thereof with two sectors 71 and 72 willincrease the perimeter of their rolling surface 10 a, 10 b so as tocompensate the wear of the rollers 27.

The invention is of course not limited to only the embodiments of thismechanism 1 described above as examples, but on the contrary encompassesall alternatives thereof. The segment support device 29 can comprise acomplementary device for pressing the segments 31 on the inner surface 4of the single-piece body 3 of the outer stator enclosure 2 so as to makeit possible to use a different stator profile and increase the volumeratio between the volume chambers 35, and the return device 50illustrated here by the dampers 50 may be of any type, on the conditionit provides contact between the guide means (27, 27 a, 27 b) and therolling surfaces (10 a, 10 b).

1. A rotary piston mechanism comprising; an outer enclosure forming astator inside which a rotary assembly forming a rotor moves, the rotaryassembly comprising: a plurality of pistons forming an articulatedpolygon, each piston delimiting a volume chamber with the enclosure, andfirst guide means arranged to cooperate with second guide meanscomprising rolling surfaces, so as to stress the polygon to perform apredetermined movement, and wherein the mechanism comprises a returndevice maintaining contact between the guide means and the rollingsurfaces.
 2. The mechanism according to claim 1, wherein the returndevice is supported by the rotary assembly.
 3. The mechanism accordingto claim 1, wherein the return device comprises a plurality of damperspositioned between a stationary point of each piston and a movablefastening point of each of the first guide means comprising a guideroller axis of rotation.
 4. The mechanism according to claim 1, whereinthe return device is supported by the outer enclosure.
 5. The mechanismaccording to claim 1, wherein the second guide means are supported bylateral flanges positioned coaxially to the outer enclosure and servingto close the latter part.
 6. The mechanism according to claim 1, whereinthe second guide means comprise two sectors, a first of which is securedto the stator enclosure and a second of which is translationally movablerelative to the first, a return device being positioned between astationary point of the first sector and a movable point of the secondsector of each of the second guide means.
 7. The mechanism according toclaim 1, wherein a segment support device is pivotably mounted along asingle axis on each apice of the polygon.
 8. The mechanism according toclaim 7, wherein the segment support device comprises a device forpressing segments on the stator enclosure.
 9. The mechanism according toclaim 7, wherein the segment support device comprises a complementarydevice for pressing segments on the stator.
 10. The mechanism accordingto claim 1, wherein at least one piston is connected by a singleconnecting rod or by several connecting rods having same hinge pins asthe transmission shaft.
 11. The mechanism according to claim 1, whereintwo adjacent pistons are connected to one another by a pivot link withan axis parallel to an axis of rotation of the rotary assembly.
 12. Themechanism according to claim 1, wherein the second guide means areturned toward a axis of rotation of the rotary assembly.
 13. Themechanism according to claim 1, wherein the rotary assembly comprises alubrication circuit mounted in a closed circuit.
 14. A pump comprising amechanism according to claim
 1. 15. An engine comprising a mechanismaccording to claim 1.